Opioid receptor mechanisms at the hypoglossal motor pool and effects on tongue muscle activity in vivo

Abstract

Opioids can modulate breathing and predispose to respiratory depression by actions at various central nervous system sites, but the mechanisms operating at respiratory motor nuclei have not been studied. This study tests the hypotheses that (i) local delivery of the mu-opioid receptor agonist fentanyl into the hypoglossal motor nucleus (HMN) will suppress genioglossus activity in vivo, (ii) a component of this suppression is mediated by opioid-induced acetylcholine release acting at muscarinic receptors, and (iii) delta- and kappa-opioid receptors also modulate genioglossus activity. Seventy-two isoflurane-anaesthetised, tracheotomised, spontaneously breathing rats were studied during microdialysis perfusion into the HMN of (i) fentanyl and naloxone (mu-opioid receptor antagonist), (ii) fentanyl with and without co-application of muscarinic receptor antagonists, and (iii) delta- and kappa-opioid receptor agonists and antagonists. The results showed (i) that fentanyl at the HMN caused a suppression of genioglossus activity (P < 0.001) that reversed with naloxone (P < 0.001), (ii) that neither atropine nor scopolamine affected the fentanyl-induced suppression of genioglossus activity, and (iii) that delta-, but not kappa-, opioid receptor stimulation also suppressed genioglossus activity (P = 0.036 and P = 0.402 respectively). We conclude that mu-opioid receptor stimulation suppresses motor output from a central respiratory motoneuronal pool that activates genioglossus muscle, and this suppression does not involve muscarinic receptor-mediated inhibition. This mu-opioid receptor-induced suppression of tongue muscle activity by effects at the hypoglossal motor pool may underlie the clinical concern regarding adverse upper airway function with mu-opioid analgesics. The inhibitory effects of mu- and delta-opioid receptors at the HMN also indicate an influence of endogenous enkephalins and endorphins in respiratory motor control.

Figures

Figure 1. Example and group data showing the location of the microdialysis probes from all the experiments in Protocols 1a–d from Study 1: modulation of genioglossus activity by μ-opioid receptor mechanisms at the hypoglossal motor pool

The top images show histological sections with the location of microdialysis site indicated by the arrow, with adjacent regions of the hypoglossal motor pool (rostral and caudal) also shown. Abbreviation: HMN, hypoglossal motor nucleus.

Figure 2. Modulation of GG activity by μ-opioid receptor mechanisms at the hypoglossal motor pool

A, example in one rat showing selective depression of respiratory-related genioglossus activity with microdialysis perfusion of the μ-opioid receptor agonist fentanyl into the hypoglossal motor pool, and the subsequent reversal of this suppression with application of the μ-opioid receptor antagonist naloxone. B, group data showing significant suppression of respiratory-related genioglossus activity with fentanyl at the hypoglossal motor pool in Protocols 1a–c. Subsequent application of naloxone led to a significant increase in genioglossus activity (Protocol 1a) that did not occur with a switch to either atropine or artificial cerebrospinal fluid (ACSF) (Protocols 1b and 1c respectively). See text for further details. *P < 0.05 compared to baseline ACSF controls; +P < 0.05 compared to the corresponding ACSF time-control experiment (i.e. Protocol 1d); #P < 0.05 from fentanyl to naloxone. Abbreviations: EEG, electroencephalogram; EMG, electromyogram; MTA, moving-time average.

Figure 3. Responses to the μ-opioid receptor agonist fentanyl at the hypoglossal motor pool were specific to the genioglossus muscle as there were no significant effects of these fentanyl applications on the amplitude of diaphragm activity, respiratory rate or blood pressure in Protocols 1a–c compared to the changes over time observed in the ACSF time-control experiments (Protocol 1d)

See text for further details.

Figure 5. Group data showing significant suppression of respiratory-related genioglossus activity with microdialysis perfusion of the μ-opioid receptor agonist fentanyl into the hypoglossal motor pool in Protocols 2a–c, and the lack of effect of co-applied atropine or scopolamine on these responses

See text for further details. *P < 0.05 compared to baseline ACSF controls; +P < 0.05 compared to the corresponding ACSF time-control experiment (i.e. Protocol 2d).

Figure 4. Location of the microdialysis probes from all the experiments in Protocols 2a–d from Study 2: continuous μ-opioid receptor stimulation at the hypoglossal motor pool with and without muscarinic receptor antagonism

Abbreviations are as for Fig. 1.

Figure 6. Responses to δ-opioid receptor stimulation at the hypoglossal motor pool

A, location of the microdialysis probes from all the experiments in Protocol 3 from Study 3: δ-opioid receptor stimulation at the hypoglossal motor pool. B, group data illustrating significant suppression of respiratory-related genioglossus activity with microdialysis perfusion of the δ-opioid receptor agonist DPDPE into the hypoglossal motor pool. *P < 0.05 compared to baseline ACSF controls; +P < 0.05 compared to the corresponding ACSF time-control experiment. See text for further details.

Figure 7. Responses to κ-opioid receptor stimulation at the hypoglossal motor pool

A, location of the microdialysis probes from all the experiments in Protocol 4 from Study 4: κ-opioid receptor stimulation at the hypoglossal motor pool. B, group data illustrating that there was a decline in genioglossus activity over the course of the experiment with microdialysis perfusion of the κ-opioid receptor agonist U-50488 into the hypoglossal motor pool, which first became statistically significant (P < 0.05) at 10 μm U-50488 (indicated by *). However, there was no significant difference in the effect of κ-receptor agonists or antagonists at the hypoglossal motor pool compared to the ACSF time controls. See text for further details.